RESPONSES OF THE SUPERFICIAL ENTORHINAL CORTEX IN VITRO IN SLICES FROM
NAIVE AND CHRONICALLY EPILEPTIC RATS.
Bear, Jonathan, Nathan B. Fountain, and Eric W. Lothman.
Department of Neurology and Neuroscience Program, University of Virginia
Health Sciences Center, Charlottesville, Virginia 22908, U.S.A..APStracts 3:0156N, 1996.
SUMMARY AND CONCLUSIONS
1. The main purposes of this study are to characterize the intracellular and
extracellular responses of cells in superficial layers of entorhinal cortex
(EC) in chronically epileptic animals, determine whether their altered
physiology is dependent on being connected to hippocampus, and investigate
whether there is evidence of augmented excitation and inhibitory interneuron
disconnection. 2. Functional connectivity was maintained between the
hippocampal area and the EC in vitro in a combined rat hippocampal-
parahippocampal slice preparation by slicing with a vibratome at a 30 degrees
angle to the base of the brain. Three groups of animals were studied: naive
animals; animals that had experienced a previous episode of (nonconvulsive)
self sustaining limbic system status epilepticus (SSLSE) induced by electrical
stimulation resulting in a chronically epileptic state; and an electrode
control group. In chronically epileptic rats and the electrode control group,
studies were done on tissue contralateral to the side of electrode
implantation. 3. Extracellular and intracellular recordings were made from the
superficial layers of EC. Neurons in the superficial layers of the EC were
activated by stimulation of the deep layers within the EC or the angular
bundle adjacent to the EC, which contains axons from EC neurons. Responses
could be elicited by antidromic and synaptic mechanisms by stimulation at
either site. In addition, a monosynaptic protocol was used which involved
direct activation of interneurons with a stimulating electrode placed near the
recording electrode in the presence of the ionotropic glutamate blockers D(-)-
2-amino-5-phosphonovaleric acid (APV) and 6,7-dinitroquinoxaline-2-3-dione
(DNQX). 4. Responses were collected over a range of stimulus intensities, from
very low to high intensities, to construct Input/Output function (I/O) curves.
Amplitudes and durations were measured at the lowest stimulus intensity that
elicited a maximum response. 5. Extracellular field potential responses from
electrode controls did not differ from naives qualitatively with respect to
morphology of field potential responses or quantitatively with respect to
response duration and amplitude. Field potential responses in tissue from
post-SSLSE rats differed markedly in morphology from naive and electrode
controls, being more complex, significantly longer in duration and decreased
in amplitude. These epileptiform responses were shortened markedly by blockade
of NMDA receptors with APV, but this manipulation did not convert responses to
a normal morphology. These responses were abolished by blockade of non-NMDA
mediated ionotropic glutamate receptors with DNQX. 6. During intracellular
recordings of neurons in slices from both control and epileptic animals,
neurons were quiescent under resting conditions, in the absence of electrical
stimulation. 7. Intracellular responses in electrode controls were identical
to naive, and together were considered "controls." In control tissue, evoked
intracellular responses were similar to those previously described and most
commonly consisted of an excitatory postsynaptic potential (EPSP) which was
partially blocked by the NMDA-receptor antagonist APV, followed by
hyperpolarizing potentials which were identified electrophysiologically and
pharmacologically as GABA A and GABA B receptor-mediated inhibitory
postsynaptic potentials (IPSPs). EPSPs were completely blocked by DNQX. 8. In
chronically epileptic tissue, evoked intracellular responses differed markedly
from responses in control animals, exhibiting all-or-none prolonged paroxysmal
depolarizing events with multiple superimposed action potentials in response
to a single shock. These depolarizing events were reduced in duration and
amplitude, but not abolished, in APV. IPSPs were not seen or markedly reduced
at all stimulus intensities. These intracellular responses never resembled
control responses. Intracellular responses correlated precisely in morphology
and duration with extracellular field potentials. 9. In chronically epileptic
tissue with the monosynaptic protocol used to directly activate inhibitory
interneurons, hyperpolarizing potentials were evoked which were
electrophysiologically and pharmacologically identified as GABA A and GABA B
receptor-mediated IPSPs and did not differ from monosynaptic IPSPs in control
animals. 10. The findings are consistent with the hypothesis that superficial
layer EC cells are hyperexcitable, as in CA1, in this model and that this is
due to augmented excitation and impaired inhibition, possibly because
GABAergic inhibitory interneurons are dormant (disconnected from excitatory
input). 11. These results constitute the first study of the EC in an animal
model of chronic temporal lobe epilepsy and provide evidence of
electrophysiologic changes outside of the hippocampus.

Received 1 June 1995; accepted in final form 27 June 1996.
APS Manuscript Number J357-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 July 1996